a vac. gauge will tell you faster if its got a burned valve or not. its possible, but rarely will a burned valve cause only 50psi of loss. most of the time its a massive loss.



and anyone with a compression gauge can tell you whether or not its a valve or not. just about every compression gauge set available (mine does at least) has an air hold adaptor that hooks to the hose, allowing you to pressurize the cylinder for doing valve spring service. use it, and listen to see where the air is coming from. granted, its a little more involved (you gotta make sure the cam is on base circle for the cylinder you are doing, etc) but its not that hard.

if its a 5.2 or 5.9 (i assume its on a dakota? or am i completely off base judging by your username?) its a damn good possibility of an injector issue. if you have one cylinder over fueling, it can wash down the cylinder and cause a low compression reading. a wet test will verify if its a ring issue or valvetrain issue. i have also seen cam lobes wear off on those engines, although its usually on cyl 1 or 2 though.